1. Field of the Invention
Embodiments of the present invention are directed to phosphor compositions in light emitting devices comprising a light emitting diode (LED) or laser diode and an excitable phosphor composition.
2. Description of the Related Art
The advent of GaN-based epitaxial structures has lead to the development of UV and/or blue (“UV/blue”) radiation sources in the form of light emitting devices, including light emitting diodes and lasers, both generally referred to herein as “LEDs.” In general, the radiation source excites a phosphor or phosphor composition comprising two or more phosphors to generate visible light in the red, green, or blue region of the spectrum. The red, green, and blue light may then be combined to make white light. The phosphor(s) transforms a portion of the UV and/or blue light into light of longer wavelength in a process known as down conversion. For example, a process of making white light whereby yellow light from a phosphor such as Y3Al5O12:Ce3+ (commonly referred to as YAG-Ce) is combined with the blue light from a blue LED has been disclosed by Nichia Chemical Company.
To form white light, the YAG-Ce phosphor converts a portion of the blue light from the blue LED chip into yellow light, which is combined with blue light from the LED that had not absorbed by the phosphor. This produces a white light with a color rendering index (CRI) of about 77, and a color temperature ranging from about 6,000 to 8,000 K. For some applications, the generation of white light by down conversion of UV/blue light from an LED to longer-wavelength light using phosphors (and then combining light from with blue LED with light emitted by the phosphor) may be a more attractive white light to the user than white light made by combining red, green, and blue light directly from a red, green, and blue LED, respectively. Such UV/blue phosphor devices, for example, enable a more widely available color range, which is important for display as well as for illumination applications. The addition of different phosphors to Nichia's yellow phosphor to convert UV/blue LED light to wavelengths other than yellow is known, and thus, it is known that the overall, combined color of the product light from an LED/phosphor system may be modified by adjusting the individual phosphors in a composition.
A difficulty often encountered when generating white light by these techniques is a variation in the quality of the white light produced as a result of statistical fluctuations that occur during the manufacturing of the blue/UV LEDs. Blue and/or UV emitting LED devices are fabricated by depositing a variety of materials in a layered fashion onto a wafer of semiconductor material. The wafer is processed to until an array of tens, hundreds, or even thousands of LEDs are made. They are then separated by a technique known as dicing to form the individual LED “chips.” But manufacturing LED chips in this fashion poses an inherent problem: they cannot all be made perfectly alike, and there is bound to be some variation among the diced LED chips. Such variations may be manifested by color output of the LED, for example, as characterized by spectral power distribution and peak emission wavelength. These quantities may vary due to fluctuations in the bandgap width of the active layer(s) of the LED(s). Another cause of variable blue/UV light output is the fact that, during operation, the power supplied to used to drive the LED may fluctuate as well.
During production, a certain percentage of LEDs are manufactured with active layers whose actual band gap width is either larger, or smaller, than that which is desired. Thus, the color output of such LEDs deviates from the desired parameters. Furthermore, even if the band gap of a particular LED does have the desired width, the power applied to the LED can change during operation. This can also cause the LED color output to depart from desired parameters. Since the light emitted by some systems contains a blue component from the LED, the color output of the LED changes as well. A significant deviation from the desired parameters may cause the color output of the system to appear non-white (i.e., either bluish or yellowish).
Past solutions to this problem have included a “binning” procedure, in which the electroluminescent characteristics of each of the blue/UV LEDs arrayed on a wafer are measured prior to dicing, after which the individual LEDs are categorized (or “sorted”) in terms of any of 1) peak emission wavelength of the light emitted by the LED, 2) peak intensity of the light emitted by the LED, and 3) by forward voltage. Binning relies on the fact that LEDs are current devices. This means that the intensity of the light emitted by the LED is regulated by the electrical current supplied to the LED, referred to as the “forward current.” Often, a series resistor is placed in the circuit proximal to the voltage source: this resister protects the LED from an excessive current overload. The value of the forward voltage depends on this series resistance, the voltage supplied to the circuit, and the desired forward current through the LED (computed from the desired intensity, since the light output is directly proportional to the forward current).
A typical commercial binning process sorts the LEDs after fabrication by any of forward voltage, peak emission wavelength, and peak emission intensity, depending on the importance of those parameters to the manufacturer. As explained above, the voltage applied to the circuit determines the current that flows through the diode, which in turn affects the intensity of the light emitted from the device. Thus, variations in portions of the circuit supporting the LED, particularly that portion which supplies power, is manifested in the “series resistance” that affects the current delivered to active layer of the LED. As demonstrated schematically in FIG. 1, an imaginary wafer has been diced to separate the individual LED circuits, and sorted according to three forward voltage groupings described generally as VF1, VF2, and VF3.
The band gap width of the LEDs' junction region determines the peak emission wavelength of the emitted light, which in turn affects color output and chromaticity. For any one value of the forward current, a range of peak emission wavelengths (e.g., color output) may be observed. This is illustrated schematically in FIG. 1. For each group of forward voltage values, sorting further proceeds by arranging LEDs into subgroups based on emission wavelength. In FIG. 1, a set of five binned peak emission wavelengths are first grouped according to the value of the forward voltage across the LED; then for each of the three forward voltage bins, LEDs are further categorized by peak emission wavelength. In the example of FIG. 1, the peak wavelength bins are centered at 452.5, 455.0, 457.5, 460.0, and 462.5 nm, respectively. The bins themselves may have the ranges: less than 450 nm, 450 to 425.5 nm, 425.5 to 455 nm, 455 to 457.5 nm, 457.5 to 460 nm, and greater than 460 nm.
Some manufacturing operations may require further refinement in LED sorting. For example, each of those bins described in FIG. 1 (which are 2.5 nm wide) may be divided into an additional five bins according to chromaticity, resulting in a total now of 75 bins. The binning process could go on indefinitely. For example, the 75 bins each having a 2.5 nm width may be further divided into three groups by brightness. In the example of FIG. 1, there are now a total of 225 bins defined for all of the LED chips originally fabricated on the hypothetical wafer.
While desirable in some instances, binning does not have to be predicated on wavelength ranges of the excitation source. In some processes, each LED chip, (or “die”) is electrically connected to an external circuit via two electrodes, and the diced LED wafer is then tested for forward voltage of the device, or light output power from the illumination system. Exemplary categories for binning by forward voltage into four bins are less than 3 volts, 3.0 to 3.2 volts, 3.2 to 3.4 volts, and 3.4 to 3.6 volts. Alternatively, when binning is based on light output power, such bin categories may be arranged as less than 8 mW, 8 to 10 mW, 10 to 12 mW, and 12 to 14 mW.
The light output of the blue/UV LED determines in part the color output of the illumination system (meaning LED plus phosphor). The color coordinate index (CIE) for a white LED produced by a blue GaN based light emitting diode (LED) providing excitation radiation to a yellow phosphor is controlled mainly by the emission wavelength of the light from the blue LED. Thus, there is a matching process involved with pairing individual LED chips with phosphors. Thus far in the industry, it may be said in general that yellow phosphors having emission wavelengths ranging from about 550 nm to about 575 nm have been selected to match blue LED wavelengths ranging from about 450 nm to about 470 nm, respectively. Such matching has achieved a desired color coordinate index, for example, of CIE (0.300, 0.300). But a large bin/sorting operation is necessary to process today's blue LED chip output into white LED-based lighting systems, largely in part because of the variation in emitting light wavelength from the blue/UV chip.
What is needed in the art is a phosphor composition designed with the ability to correct, or “self-adjust” the chromaticity of the light it emits in response to wavelength/energy variations in the excitation radiation.